In a mixing bow, combine flour, sugar, and salt.
In a saucepan, melt butter. Add cocoa. Stir together.
Add boiling water, allow mixture to boil for 30 seconds, then turn off heat. Pour over flour mixture, and stir lightly to cool.
In a measuring cup, pour the buttermilk and add beaten eggs, baking soda, and vanilla. Stir buttermilk mixture into butter/chocolate mixture. Pour into sheet cake pan and bake at 350-degrees for 20 minutes.
While cake is baking, make the icing. Chop pecans finely. Melt butter in a saucepan. Add cocoa, stir to combine, then turn off heat. Add the milk, vanilla, and powdered sugar. Stir together. Add the pecans, stir together, and pour over warm cake.
Cut into squares, eat, and totally wig out over the fact that youâ€™ve just made the best chocolate sheet cake. Ever.